1. Field of the Invention
The present invention generally relates to satellite communications systems, and more particularly to an earth terminal for such systems which is sufficiently reliable and so configured that unattended operation for extended periods of time is possible.
2. Description of the Prior Art
Communication satellite earth terminals have evolved from the experimental-operational designs of the early 1960's to wideband commercial designs in present use. These terminals have been supplied by a number of commercial sources to meet the transmission performance required by the expanding system demands. They have been built to commercial engineering standards, and though long life has been specified, reliability has not been emphasized.
The existing international satellite communications network is currently designed as a system of trunk links using large earth terminals needing considerable on-site manpower. The typical earth terminal is operated by an average of 30 people. The next stage of this evolving industry is the development of domestic and regional systems in which the number of earth terminals is greatly expanded. When the growing and changing needs of these systems are examined, it becomes apparent that there is a need for a reliable earth terminal designed to permit unattended operation.
Presently, satellite communications earth terminals incorporate certain design features which mitigate against reliability and unattended operation. Among these are autotracking antennas with rapid motion and complete sky coverage. Such antennas typically consist of a parabolic shaped reflector Cassegrain-fed antenna with tracking feed mounted on a servo-driven pedestal and a tracking receiver. The choice of this type of antenna has been dictated in the past by large variation in satellite station-keeping and the need to point to alternate satellites spaced at significant angles to provide for communication reliability and avoid sun outages.
Because of low noise requirements in satellite communications systems, cryogenically cooled parametric amplifiers are usually used as preamplifiers in earth terminal receivers. Such amplifiers are highly complicated requiring refrigerators, dewars, switches, and additional monitoring equipment. Reliability of the cryogenic parametric amplifiers is relatively low due primarily to the refrigerator, and failure is usually catastrophic resulting in long outages. Failure of a cooled stage or a refrigerator failure could be overcome by switching to a redundant unit. However, since both units would have been running continuously since the last maintenance, the probability of survival of one of the two is not greatly enhanced.
Current systems employ a transmitter using a single high-powered wideband amplifier covering the entire band of operation. While this type of transmitter has the advantage of simplicity and low cost, it suffers significant disadvantages. First of all, the power output of the amplifier must be equal to the largest requirement of the combined aggregate of the channels of the particular earth terminal. This necessitates large high voltage power supplies and water cooling. Second, the intermodulation and crosstalk levels imposed by communication requirements presently require the high-power tube to be operated with an output backoff of approximately 7 dB. Furthermore, with additional increases in channel capacity per carrier, crosstalk dominates resulting in the requirement for an even higher powered amplifier.